The invention relates to a method for antenna selection control in a radio receiver and to an arrangement for antenna selection control in a radio receiver.
In the case of radio-frequency telecommunication in the gigahertz region, because of the propagation characteristics the transmission conditions are frequently very different within a small spatial region (centimeter region). The consequence of this is that in the case of moving systems the transmission conditions fluctuate strongly in time even at low speeds of, for example, approximately 1 m/s. In order to be able to achieve transmission which is as good as possible during movement even in regions with poor transmission conditions, a second, spatially offset antenna is installed at the base station, which second antenna, because of the spatial separation, provides other transmission conditions and to which it is possible to switch in this case (antenna diversity). Such an antenna control is provided in essence in the "Digital European Cordless Telecommunications System" (DECT) Standard (draft prETS 300 175-3: 1991, page 183/184). An antenna switch-over is provided in this standard on the basis of the quality of reception in the following cases:
a) result of the A- and X-CRC; PA1 b) conditions of the S- and Z-fields; PA1 c) radio signal strength; and PA1 d) other suitable parameters.
The "burst-mode two-way communications system" has already been disclosed in European Patent Application EP 0 213 780 A2 (Application No.: 86306015.8). By contrast with the arrangement according to the invention, the known system has a plurality of transceiver devices having one threshold value arrangement each, which are connected to a switching element for switching over to one of two antennas. The threshold value arrangements realize a prescribed absolute threshold value (page 3, column 3, lines 55/56). The threshold value is, however, also to be adjustable from outside by means of software, and thus to be optimizable, it remaining open in the published European Patent Application how such an adjustment and optimization would have to be performed (column 3, lines 57/58; column 4, line 41). In particular, it also remains open whether such an optimization is performed by a single adjustment of the threshold value, for example during manufacture or upon commissioning of the apparatus, or is also performed in continuous operation. Obviously, only a single adjustment of the threshold value is provided, since it is stated in the European Patent Application that in practice the threshold value corresponds to that value at which reception is still just adequate ("worst-case service"), generally 15 dB below an average field strength at maximum range (column 4, lines 41-46).
In the known system, signal fades (drops in field strength) are detected by means of a rapid rate of change in the received field strength, this parameter being compared with values averaged over a plurality of time periods (column 4, lines 47-56). However, the averaging, provided for in this case, requires substantial computational outlay and a corresponding computing time. There is no provision in the known system to change the threshold value with rising field strength values.
Specifically, if appropriate systems contain two antennas, but only one receiver, there is the substantial problem for the control of the antenna switch-over that the quality of the signals received at both antennas cannot be measured simultaneously. The measurements have to be carried out one after the other in order to decide at which antenna signals of better quality of reception can be received. In this case, there is the difficulty, on the one hand, that a measurement at one antenna is bound up with the reception of a burst at this antenna, and that the measurement result "poor quality of reception" can lead to a loss of data. On the other hand, a further problem consists in that, depending on the speed of movement of the actual device or of a corresponding device to which there is a radio connection; and on the burst rate, the measurement result is valid only for a few bursts. A further difficulty consists in that the receiving conditions are not correlated with regard to the two antennas, with the result that the selection of the switch-over instant is made more difficult.
Disclosed in the printed publication: Proceedings of Globecom. '90, Dec. 2-5, 1990, New York (US), pages 1013 to 1017; L. B. Lopes: "On the radio link performance of the digital European cordless telecommunications (DECT) system" is a structure of a simulated DECT receiver in the case of which an antenna diversity controlled by field strength measurement improves the coverage (range) and the capacity of DECT cordless telephones.
The printed publication: Patent Abstracts of Japan, Volume 011, No. 231, Jul. 28, 1987 in conjunction with JP-A-62047222 discloses an antenna diversity receiver in the case of which the antenna diversity effect is improved by changing a diversity switching threshold as a function of changes in the field strength of the received signal. The first step for this is to compare a first field strength value measured at a first antenna with a stored threshold value. If in the case of this comparison the measured field strength value is lower than stored threshold value, a second antenna is activated. If, however, the measured field strength value is higher than equal to the stored threshold value, the first antenna remains activated as before. In the case of the antenna switch-over, the previously stored threshold value is replaced at the same time by the measured field strength value. The result of this is that the threshold value is also continuously reduced in the case of a gradual lowering of the field strength value. The antenna diversity receiver continues to be constructed in such a way that whenever the measured field strength value exceeds the stored threshold value by a certain value, the threshold value stored up to that point is updated.